I have a 67 gt I got almost 2 weeks ago and am sorting out some issues. When replacing the signal switch I discovered that the wiring harness has been replaced with a 68 harness. When looking under the dash I saw several plugs not connected to anything. What's the best resource for narrowing down what these plugs are for by wire color, number of wires in the plug and color? Am I better off fixing that as is or going back to a 67 harness?

Not every plug is used on every car. The wiring harness is generally set up to handle just about any possible option that may be added to any given car. If an option is not on a car then the plug will remain unused. There are a few options that have their own separate wiring harness as well, the wiring harness to the additional speakers when an AM/FM or AM/8 Track radio is in a car is an example of a separate harness. Another example is that most of the wiring harnesses will have a flat yellow plug with 3 female connections. This plug is where accessory items will receive their power. If the car does not have any of those accessories then there will be nothing plugged into that yellow plug.

It's definitely an xr7 dash.. as it has the tach and the courtesy light switches with the woodgrain front... Not sure if the wiring harness is from an xr7 as well... I just know the harness is from a 68